1. Technical Field
The present disclosure relates to an image sensor including at least two photosites separated by an isolation region and a method for fabricating the same.
2. Description of the Related Art
Camera modules typically include a lens, an infrared filter, and an image sensor in the form of a semiconductor chip including several photosites, one or more photosites corresponding to a pixel of a captured image. The photosites are generally arranged in a matrix and may include for example photodiodes, photogates, or photoconductors. In general, above the photosites are distributed a mosaic of microlenses to converge incident light rays on the photosites and a mosaic of color filters to filter the light.
An image sensor uses the photoelectric effect to transform light information into an electrical signal. A photon of an incident light ray penetrates into the material of the photosite, and if the energy of the photon is equal to or greater than the material's energy bandgap, an electron-hole pair is generated. The number of electrons per photosite may be read as an electronic signal and used to supply light information for the image. Photons with short wavelengths have a shallow penetration depth, such as 0.4 μm for wavelengths of 450 nm (blue light), and photons with long wavelengths have a deeper penetration depth, such as 2.4 μm for wavelengths of 600 nm (red light).
The performance of an image sensor depends on many factors, such as the quantum efficiency (the number of electrons generated and read per number of incident photons), the dark current (the amount of current generated even when the photosites are not receiving incident light rays), and the crosstalk (optical or electrical). Optical crosstalk between photosites occurs when the photons of an incident light ray are absorbed by a photosite other than the one that should absorb them. Electrical crosstalk occurs when an electron is generated in one photosite and migrates to another photosite. Isolation regions are therefore commonly provided between photosites to prevent optical and/or electrical crosstalk.
The isolation region may be in the form of a deep trench isolation (“DTI”) with a depth as great as possible taking into account the penetration distance of the photons with the longest wavelength, and lined with an electrically insulating liner. In this manner, generated electrons cannot pass under or through the isolation region from one photosite to another, reducing electrical crosstalk. The deep trench may further be filled with an optically isolating material such as oxide to create an “optical waveguide” reducing optical crosstalk by means of total internal reflection of light rays, so that they remain within a photosite.
FIG. 1 is a cross-sectional view of a part of a conventional image sensor IS1 including, from bottom to top as illustrated, a substrate 10 including two photosites S1, S2; an insulator layer 70; a stack 80 of dielectric layers 80-1 to 80-N, each layer including electrically conductive lines 90-1 to 90-N to extract electrical information from and power the photosites; color filters F1, F2; and microlenses L1, L2. Isolation region 11 extends between photosites S1, S2, and includes a deep trench 15 made in the substrate 10. The trench 15 has its bottom and sidewalls covered by a thin insulating liner 20, such as a dielectric material, and is filled with an optical isolator 40, such as an oxide.
FIG. 1 schematically shows three incident light rays R1, R2, R3. Light ray R1 passes through filter F1, penetrates into photosite S1, and generates an electron El in photosite S1. Light ray R2 also passes through filter Fl and generates an electron E2 in photosite S1, which remains within photosite S1 due to the deep trench 15. Light ray R3 passes through filter F2, but has an incident angle such that instead of generating an electron directly in photosite S2, it reflects off the optical waveguide formed between the substrate 10 and the isolator 40. Light ray R3 returns to photosite S2, where it generates an electron E3.
Interfacial traps are generally present at the atomic layers closest to the interface between the trench 15 and the substrate 10. These traps are caused by cutting the crystalline lattice of the substrate 10 to form the trench 15. These traps may bond with generated carriers (electrons and holes). The depth of the deep trench 15 causes a larger interface, thus a greater number of traps, increasing the dark current.